Moura, M., Barbosa, J., Pinto, I., Leça, N., Cunha-Silva, S., Verza, A.E., Pedroso, P.D., Lemaire, S., Duro, J., Simoes-da-Silva, M., Oliveira, A., Reis, R., Nilsson, J., Sunkel, C., Musacchio, A., Bollen, M., Conde, C. (2025). Polo kinase inhibits protein phosphatase 1 to promote the spindle assembly checkpoint and prevent aneuploidy.  Curr. Biol. 35(21): 5289--5307.e8.

FBrf0263768

Research paper

Protein phosphatase 1 (PP1) is essential for spindle assembly checkpoint (SAC) silencing and mitotic exit, but its regulation during mitosis remains ill-defined. Here, we demonstrate in vitro and in Drosophila cells that the mitotic kinase Polo phosphorylates PP1α87B at a conserved residue (T286) within a pocket implicated in the recognition of RVxF-containing target proteins. Phosphorylation of T286 inhibits PP1α87B binding to the RVxF motif of the SAC kinase MPS1, dampening the dephosphorylation of the MPS1 T-loop. Phosphorylation of T286 is dynamically regulated during mitosis. It occurs at unattached/tensionless kinetochores and decreases as chromosomes congress. Expression of phosphomimetic PP1α87B[T286D] prevents MPS1 inactivation in metaphase and causes a SAC-dependent delay of anaphase onset. Conversely, an unphosphorylatable PP1α87B[T286A] mutant impairs MPS1 activation at unattached kinetochores and weakens the SAC. In vivo, larval neuroblasts expressing PP1α87B[T286] phosphomutants exhibit chromosome mis-segregation and aneuploidy. Thus, our findings identify Polo-mediated phosphorylation of PP1α87B as a critical regulatory strategy that fine-tunes phosphatase activity to ensure a robust and timely SAC and prevent genome instability.